US published patent application 2004/0074639 describes a logging system for use in a wellbore wherein a tubular conduit extends from surface into the wellbore. The tubular conduit contains a body of wellbore fluid. A pressure pulse device is arranged together with a logging tool string within the same conduit. The pressure pulse device generates pressure pulses in the body of the wellbore fluid, which pressure pulses represent data communicated by the logging tool string to the pressure pulse device during logging of the logging tool string. These pressure pulses are detected by a control system at surface.
The operator is thereby in a position to evaluate the functioning of the logging tool string during the logging operation, and to take appropriate measures at an early stage if necessary.